DEEP IMPACT:
FIRST LOOK INSIDE A COMET!

What's deep inside a comet?
Comets are time capsules that hold clues about the
formation and evolution of the solar system. They
are composed of ice, gas and dust, which is the
primitive debris from the solar system's earliest
and coldest formation period, roughly 4.5 billion
years ago. Deep Impact, a NASA Discovery Mission,
is the first space mission to try to probe beneath
the surface of a comet and reveal the secrets of
its interior.

On July 4, 2005, the Deep Impact spacecraft targeted
Comet Tempel 1 with its 370-kg (~820-lbs) copper
impactor. Upon impact, scientists predicted a crater
ranging in size from that of a house to that of
a football stadium. Scientists hoped ice and dust
debris would be ejected from the crater revealing
fresh material beneath. Sunlight reflecting off
ejected material was measured by the instruments
onboard the spacecraft, and by other space- and
ground-based observatories watching the event. Images
from the Deep Impact cameras and spectrometer were
sent to Earth covering the approach, the impact
and its aftermath. The effects of the collision
with the comet were also observable from certain
locations on Earth. The data will be analyzed and
combined with that of other NASA and international
comet missions. Results from these missions will
lead to a better understanding of both the solar
system's formation and implications of comets colliding
with Earth.

The Mission
Planning and design for the Deep impact mission
started in November 1999. In January 2005, a Delta
II rocket launched the Deep Impact spacecraft toward
the comet. The spacecraft consists of two main parts-
the impactor and the larger flyby vehicle. The combined
spacecraft approached Tempel 1 and collected images
of the comet before the impact. In early July 2005,
24 hours before impact, the flyby spacecraft pointed
high-precision tracking telescopes at the comet
and releaseed the impactor on a course to hit the
comet's sunlit side.

The impactor is a battery-powered spacecraft that
operated independently of the flyby spacecraft for
just one day. The impactor is more than a cannonball.
After its release, it takes over its own navigation
and maneuvers into the path of the comet. A camera
on the impactor captured and relayed images of the
comet's nucleus just seconds before collision. The
impact was not forceful enough to make an appreciable
change in the comet's orbital path around the Sun.

After release of the impactor, the flyby spacecraft
maneuvered to a new path that, at closest approach
passed 500 km (300 miles) from the comet. The flyby
spacecraft observed and recorded data about the
impact, the ejected material blasted from the crater,
and the structure and composition of the crater's
interior. The flyby spacecraft took additional data
from the other side of the nucleus to look for changes
in the comet's activity.

Comet Tempel 1
Comet Tempel 1 was discovered in 1867 by Ernst Tempel.
The comet has made many passages through the inner
solar system orbiting the Sun every 5.5 years. This
makes Tempel 1 a good target to study evolutionary
change in the mantle, or upper crust. Comets are
visible for two reasons. First, dust driven from
a comet's nucleus reflects sunlight as it travels
through space. Second, certain gases in the comet's
coma, stimulated by the Sun, give off light like
a fluorescent bulb. Over time, a comet may become
less active or even dormant. Scientists are eager
to learn whether comets exhaust their supply of
gas and dust to space or seal it into their interiors.
They would also like to learn about the structure
of a comet's interior and how it is different from
its surface. The controlled cratering experiment
of this mission is an attempt to answer these questions.

Technical Implementation
The flyby spacecraft carried a set of instruments
and the impactor. Two instruments on the flyby spacecraft
were designed to observe the impact, crater and
debris with optical imaging and infrared spectral
mapping. The flyby spacecraft used an X-band radio
antenna (transmission at about eight gigahertz)
to communicate with Earth as it also communicated
with the impactor on a different frequency. For
most of the mission, the flyby spacecraft communicated
through the 34-meter antennae of NASA's Deep Space
Network.

The image was taken July 9 2005, 4:01 UT at
the
KPNO 4-m telescope, using the MOSAIC detector.
(Tony Farnham)